1. Field of the Invention
This invention relates to a dual circuit load-conscious-type hydraulic pressure control valve apparatus, and particularly to that for adjusting in a stable manner the liquid output pressure in a master cylinder used in a brake system of a vehicle, at a given ratio based on the load of the vehicle, even when the wheels are laden to rebound.
2. Description of Related Art
A hydraulically pressure control valve apparatus of this type, which is installed in a hydraulic channel between a master cylinder of a brake system and a brake cylinder of the rear wheels, exerts braking force equally on the front wheels and the rear wheels when the required braking force is low at the start of braking, while exerting lower braking force on the rear wheels than on the front wheels when the required braking force is over a given level, thereby preventing locking of the rear wheels. In vehicles having high load capacity, such as trucks, since the braking force which causes locking of the wheels varies depending on the load, the apparatus is adapted for controlling the cut-in pressure which triggers reduction of braking force in the rear wheels in such a way that the cut-in pressure is increased when the load is high while it is decreased when the load is low.
For example, heretofore, by using a pair of plungers, each independently controlled by the hydraulic pressure provided by a master cylinder in a brake system, respective valves are opened and closed, thereby reducing the hydraulic pressure at a given ratio, and transferring the pressure to each brake cylinder of the left and right rear wheels. This conventional mechanism is disclosed, for example, in U.S. Pat. No. 5,246,277 to Yamakoshi.
However, the load-conscious hydraulic pressure-controlling braking device disclosed in the Yamakoshi reference has the following drawbacks:
1) A clamping member for affixing an equalizer to a load detection lever at a given location is required, in order to prevent the equalizer from being dislocated when the load detection lever moves widely. Further, assembling of the braking device is complicated.
2) A rattling noise is generated due to gaps between the clamping member, the load detection lever, and the equalizer. Frictional wear is also generated due to vibration of these members.
3) In the case of circuit malfunctions, braking force becomes insufficient because the cut-in pressure in the other normal circuit remains the same.
4) The rear wheels are lifted up, so-called rear lift, as a result of the load moving towards the front wheel when the brakes are applied so fast, whereby the load transmission spring is freed from the load, and accordingly the detection lever is freed from the load. In reaction, the detection lever pushes a plunger for an instant, thereby destabilizing the characteristics of response to liquid pressure.